1. Field of the Invention
The present invention relates to an optical head for an optical disk recording/reproducing apparatus, the optical head converging a beam of light emitted from a light source and irradiating the converged beam of light onto a recording medium, as well as guiding information light reflected by the recording medium into a photo detector.
2. Description of the Related Art
The inventors of the present invention know an optical disk recording/reproducing apparatus which converges a beam of light onto a recording medium formed on an optical disk and thereby records data on and reproduces the data from the optical disk. Particularly, the above-mentioned optical disk recording/reproducing apparatus employs an optical magnetic disk, because the optical magnetic disk is a high-density memory device with a large-capacity and capable of storing data erasably.
When data is recorded on an optical magnetic disk, a beam of light emitted from a semiconductor laser is focused and irradiated onto a recording medium in the form of a beam spot to raise the temperature of the area of the recording medium irradiated by the beam spot to the Curie point or above. The area of the recording medium whose temperature is high loses a coercive force and is magnetized in the direction parallel to the direction of an external magnetic field applied thereto. Thereafter, the irradiation of the light beam is suspended while an application of the external magnetic field continues. As a result, the temperature of the area irradiated by the beam spot is lowered below a temperature of the Curie point, and the area remains to be magnetized in the above-described direction, thereby the data is recorded.
When the data is reproduced from the optical magnetic disk, a beam of light is emitted at a level low enough not to heat the recording medium to a temperature higher than the Curie point, and this beam of light is focused on the recording medium. At that time, Kerr effect (magneto-optical effect) occurs on the beam of light focused on the recording medium and the plane of polarization of the light is thus rotated through an angle depending upon the direction in which the irradiated area is magnetized. Thus, the data can be reproduced by detecting the direction in which the irradiated area is magnetized from the direction in which the plane of polarization is rotated.
On the optical magnetic disk, information tracks and guide tracks are alternately formed in a concentric or spiral fashion with spaces of about 1.6 .mu.m. The data can be recorded on and reproduced from a desired position on the optical magnetic disk by utilizing the guide track. In order to achieve accurate data recording and reproducing, the beam of light must be controlled such that it is focused on the recording medium and follows the guide track, i.e., focusing control and tracking control must be performed. Hence, the optical head detects a focusing error signal and a tracking error signal, and drives an objective through an objective actuator in the focusing and tracking directions on the basis of the detected servo error signals.
FIG. 1 shows one example of the above-mentioned optical head. In this optical head, linearly polarized rays of light emitted from a semiconductor laser 21 are converted into a beam made up of parallel rays of light by a collimator 22, and the resulting beam passes through a polarization beam splitter 23. Thereafter, the beam is focused and irradiated by an objective 24 on a recording surface 25 of a recording medium in the form of a beam spot. When the rays of light are reflected by the recording surface 25 of the recording medium, they are modulated such that the plane of polarization thereof is rotated through an angle depending on the direction in which the area irradiated by the beam spot is magnetized and becomes information light. The modulated information light passes through the objective 24 again and is made incident on the polarization beam splitter 23 from a surface 23a thereof. The incident light is reflected by a splitting surface 23b and then propagates in a direction indicated by a1.
The information light which propagates in the direction a1 is split into two components by a splitting surface 23c. The information light which is directed in a direction indicated by b1 emerges from a surface 23d and then reaches a servo error signal detection optical system which consists of a condenser 26, a cylindrical lens 27 and a photo detector 28. The servo error signal detection optical system respectively detects a focusing error signal and a tracking error signal by the astigmatism method and the push-pull method from the information light which passes through the optical system.
The information light which is directed in a direction indicated by b2 emerges from the beam splitter 23 through a surface 23d' thereof and is then demodulated by a read signal detection optical system which consists of a halfwave plate 29, a condenser 30, a polarization beam splitter (polarizer) 21, and a pair of photo detectors 32 and 33. The read signal detection optical system includes the pair of photo detectors so that the noises having the same phase can be cancelled by operating the differential output of these photo detectors and a read signal having a high quality can thus be obtained.
However, since the optical head of the abovedescribed type contains two detection optical systems, that is, the servo error signal detection optical system and the read signal detection optical system, it requires a large number of optical components and thus have a complicated structure.
This problem of the above-mentioned optical heads has been solved by an optical head which is disclosed in Japanese Patent Application Laying Open No. 63-187440 and which utilizes a modified Wollaston prism disclosed in Japanese Patent Application Laying Open No. 63-113503 to simplify the configuration.
In the above-mentioned optical head, the rays of light emitted from the semiconductor laser 21 pass through the same path and are focused onto the recording surface 25 of the recording medium, as shown in FIG. 2. The information light reflected by the recording surface 25 of the recording medium emerges from the beam splitter 23 through the surface 23c and is then made incident on a modified Wollaston prism (analyzer) 34.
The modified Wollaston prism 34 splits the incident rays of light into three components. One of the three components is a central beam whose intensity remains the same as that of the incident light, and two of the three components are a left beam and a right beam, of which each intensity is varied in accordance with the polarized state of the incident light.
The photo detector 28 consists of six elements in which four elements correspond to the central beam while two elements correspond to the right and left beams, respectively. The three beams emerging from the Wollaston prism 34 are transformed into rays of beam exhibiting astigmatism by the passing through the condenser 26 and the cylindrical lens 27. A servo error signal is detected from the output of the four elements corresponding to the central beam, and a read signal is detected by operating the differential output of the remaining two elements.
Thus, in the above-described type of optical head, the single optical system can be used as both the servo error signal detection optical system and the read signal detection optical system, and the photo detector can be constructed by one component which has six elements. In consequence, the optical system can be simplified.
Now a principle of detecting the servo error signals will be explained. FIGS. 3 to 8 show, in a simplified fashion, the optical path from the recording surface 25 of the recording medium to the servo error detection optical system.
The rays of light emitted from the semiconductor laser (not shown) are focused by the objective 24 on the recording surface 25 of the recording medium in the form of a beam spot. When the light is reflected by the recording surface 25 of the recording medium, it possesses information, and the light having information passes through the objective 24, the condenser 26 and the cylindrical lens 27 in that order and thereby exhibits astigmatism. This light is received by the light detector 28 having four light receiving portions 28a, 28b, 28c and 28d which are divided by the two boundary lines inclined by 45.degree. relative to the generatrix of the cylindrical lens 27. Let the intensities of light received by the light receiving portions 28a, 28b, 28c and 28d be Sa, Sb, Sc and Sd respectively, then the focusing error signal (FES) is given by FES=(Sa+Sc)-(Sb+Sd), and the tracking error signal (TES) is given by TES=(Sa+Sb)-(Sc+Sd).
When the beam of light is in focus on the recording medium, a beam spot 42 formed on the photo detector 28 is circular, as shown in FIG. 4. In consequence, the intensities Sa, Sb, Sc and Sd of the light received by the four light receiving portions 28a, 28b, 28c, and 28d are the same, i.e., FES=0.
When the distance between the recording surface 25 of the recording medium and the objective 24 is too short, as shown in FIG. 5, the photo detector 28 detects a beam spot 42 which is elongated in a direction b0 which is parallel to the generatrix of the cylindrical lens 27, as shown in FIG. 6. In consequence, FES &lt;0.
When the distance between the recording surface 25 of the recording medium and the objective 24 is too long, as shown in FIG. 7, the photo detector 28 detects an elliptical beam spot 42 which is elongated in a direction a0 which is perpendicular to the generatrix of the cylindrical lens 27, as shown in FIG. 8. In consequence, FES&gt;0.
Thus, it is possible to determine whether the distance between the recording surface 25 of the recording medium and the objective 24 is appropriate, too long or too short by using the sign of the value of the focusing error signal. This type of focusing error signal detection method is called the astigmatism method.
The beam spot formed on the photo detector 28 has a shadow portion 43 (hereinafter referred to as a diffraction pattern) due to a diffraction caused by an information track 25a, as shown in FIGS. 9 to 14.
This diffraction pattern 43 occurs even when the beam of light is in focus, as shown in FIGS. 3 and 4. The diffraction pattern 43 varies, as shown in FIGS. 10, 12 and 14, in accordance with a shift in the positional relation between a beam spot 41 formed on the recording surface 25 of the recording medium and the information track 25a, a guide track 25b, as shown in FIGS. 9, 11 and 13 respectively.
When the beam spot 41 is at the center of the information track 25a, as shown in FIG. 11, the corresponding diffraction pattern 43 is symmetrical with respect to the line X--X which separates the light receiving portions 28a and 28b from the light receiving portions 28d and 28c, as shown in FIG. 12. In consequence, TES=0. However, when the beam spot 41 deviates from the center of the information track 25a, the corresponding diffraction pattern 43 shows asymmetry. In consequence, TES.noteq.0.
Thus, it is possible to determine whether or not the positional relation between the beam spot formed on the recording surface 25a of the recording medium and the information track 25a is appropriate from the sign of the value of the tracking error signal, and if the positional relation is not appropriate, a deviational direction of the beam spot 41 can be determined from the sign of the value of the tracking error signal as well. This type of stracking error signal detection method is called the push-pull method.
In the above-described optical head which employs the modified Wollaston prism, or in the optical head which does not use the modified Wollaston prism, the polarization beam splitter 23 is mounted on a housing in a state in which it is elastically supported by a spring or in which it is fixed by an adhesive.
Although mounting of the polarization beam splitter 23 may be performed highly and precisely at the stage of manufacture of the optical head, a position or an angle at which the polarization beam splitter 23 is mounted may change with time due to the changes in the temperature or various other factors associated with the mounting precision of the polarization beam splitter 23.
When the polarization beam splitter 23 is displaced from its original position indicated by a solid line in FIG. 15 (where the offset of the servo error signal is zero) to a position which is separated from the original position by .DELTA.d and which is indicated by a virtual line, as shown in FIG. 15, the optical axis of the rays of light propagating toward the servo error signal detection optical system is displaced by .DELTA.d, as shown by the virtual line.
When the polarization beam splitter 23 is inclined by an angle .DELTA..theta., as shown in FIG. 16, the optical axis of the rays of light which are directed toward the servo error signal detection optical system inclines at an angle 2.DELTA..theta. which is twice the angle .DELTA..theta., as shown by the virtual line. As a result, the beam spot formed on the photo detector 28 deviates, and a great offset occurs in the servo error signal, thereby the accurate recording and reproducing of the data become difficult to be performed.